Work out grade change length instantly with clear inputs, formula shown and shareable results.
Vertical curve length is specified as a K value, the length per percent of algebraic grade change, which encodes the sight distance requirement for the design speed. Multiplying K by the algebraic difference gives the minimum length, subject to an absolute minimum for appearance and drainage, typically 0.6 times the design speed in metres.
Minimum length
L = K x algebraic difference in grade, subject to an absolute minimum
Curve type
A falling algebraic difference gives a crest curve, a rising one a sag curve
Preliminary estimate only. Vertical curve design must follow the geometric design standard adopted by the road authority.
L = K x algebraic difference in grade, subject to an absolute minimum. Vertical curve length is specified as a K value, the length per percent of algebraic grade change, which encodes the sight distance requirement for the design speed.
Crests are governed by stopping sight distance over the hump. Sags are governed by headlight throw at night and by passenger comfort, so the required K values differ for the same speed.
This calculator takes 4 inputs: Incoming grade, Outgoing grade, K value for the design speed, Absolute minimum curve length. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.